Researchers exploring the ocean’s deepest reaches are accustomed to seeing oddities, from fish as fragile as tissue paper to translucent sea cucumbers. But even veteran deep-water scientists are shocked by the latest discovery at the very bottom of the sea: toxic pollution – lots of it.

Tiny shrimp-like crustaceans living in the Mariana Trench, one of the world’s most remote habitats, are laced with staggering levels of industrial chemicals, according to a new study. Scientists found that some of the Mariana crustaceans are more contaminated with the harmful pollutants called PCBs than crabs living in waters fed by one of China’s most polluted rivers.

The scientists found high levels of flame retardants in the bodies of similar crustaceans living in the Kermadec Trench, the world’s fifth-deepest and more than 4,000 miles from the Mariana.

Humans have left a “footprint in the deepest places in the world,” said study co-author Alan Jamieson of Britain’s Newcastle University. “Not only are (the pollutants) in every single sample, regardless of species, depth, trench, whatever, the concentrations are extraordinarily high. That was a big surprise.”

Anything that lives in in the Mariana Trench – the world’s deepest, at more than 6 miles – and other underwater gashes in the Earth’s surface is a tough customer. It must thrive in utter darkness and under crushing pressures of seven tons per square inch, enough to pulverize costly scientific gear. During one of Jamieson’s research voyages, an automated submersible scanning the Mariana trench imploded underwater, and only bits of debris floated to the surface.

In these unforgiving conditions, the shrimp-like scavengers called amphipods thrive by snatching any bit of food that floats by. But that voracity can backfire. The contaminants in the amphipods’ bodies probably came from the food they ate, the scientists write in this week’s Nature Ecology & Evolution.

It’s not yet clear whether the poisons do amphipods any harm, but ill effects would not be a surprise. Cancer and other health problems in animals have been linked to PCBs, and their manufacture was phased out decades ago. The flame retardants found at high levels in the Kermadec amphipods belong to a class of chemicals called PBDEs. Lab tests have tied PBDEs to thyroid problems, and many countries are phasing out their use.

The researchers did not trace the contaminants to their origins, but landfills are one possibility. Poorly maintained landfills could leach chemicals into rivers, which would funnel them to the ocean. The Mariana Trench is in the western Pacific, off shore from highly industrialized countries such as Japan, so perhaps industrial accidents help account for the PCBs there. Once the chemicals descend into the trenches, there’s nowhere else for them to go.

Humans may think that anything dumped in the ocean “magically disappears,” Jamieson said. “It doesn’t magically disappear. It ends up somewhere. … There’s no such thing as out of sight, out of mind.”

Other scientists said the new findings have sobering implications. The study confirms “human activities can penetrate to remote habitats,” agreed Henry Ruhl of Britain’s University of Southampton, who was not associated with the research.

The discovery of these poisons in a supposedly isolated setting “is surprising and quite alarming,” said Katherine Dafforn of Australia’s University of New South Wales, who wrote a commentary accompanying the new study. “Chemicals that were produced, subsequently regulated and then largely eliminated before I was even born have continued to persist, and now we find evidence of them even in our deepest oceans.”